#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<math.h>
#include<iostream>
using namespace std;
int main()
{
	double a, b, c;
	scanf("%lf %lf %lf", &a, &b, &c);
	if (a + b > c || b + c > a || a + c > b&&a>=0&&b>=0&&c>=0&&a<=1000&&b<=1000&&c<=1000)
	{
		double p = (a + b + c) / 2;
		double s = sqrt(p * (p - a) * (p - b) * (p - c));
		printf("%.1f", s);
	}
	return 0;
}